The present invention relates to navigation satellite receivers, and more particularly to methods and systems for infrequently supporting an otherwise completely autonomous client on a computer network.
The global positioning system (GPS) is a satellite-based radio-navigation system built and operated by the United States Department of Defense at a cost of over $13 billion. Twenty-four satellites circle the earth at an altitude of 20,200 km, and are spaced in orbit such that at any time a minimum constellation of six satellites is visible to any user. Each satellite transmits an accurate time and position signal referenced to an atomic clock. A typical GPS receiver locks on to this atomic clock, and then can very accurately measure the time delay for the signal to reach it, and the apparent receiver-satellite distance can then be calculated. Measurements from at least four satellites allow a GPS receiver to calculate its position, velocity, altitude, and time.
High-sensitivity GPS receivers are a problem when the initial time or frequency uncertainty is large. Finding signal energy when the signal energy is extremely faint requires making smaller steps and dwelling at each step longer. So having a better initial estimate of the local reference oscillator can improve time-to-first-fix.
GPS receivers with signal levels better that xe2x88x92145 dbm can readily lock onto a strong GPS satellite vehicle (SV) to decode the NAV-data. Such yields the SV ephemeris and position. After than, the total pseudorange needs to be formed from the hardware codephase. Conventional GPS receivers determine the integer millisecond and so-called z-count.
When signal levels are roughly no better than xe2x88x92145 dbm to xe2x88x92150 dbm, a practical high-sensitivity GPS receiver can employ pattern-matching tricks to get a z-count or integer millisecond for an anywhere-fix.
It is therefore an object of the present invention to provide a high-sensitivity GPS receiver that can operate with signal levels under xe2x88x92150 dbm.
Briefly, a navigation-satellite receiver embodiment of the present invention depends on a network server to occasionally provide key pieces of information needed during its initialization. The navigation-satellite receiver critically maintains its position uncertainty, sigmaPos, to under 150-km. Therefore, at least every five minutes, the navigation-satellite receiver uses a network connection to download all the ephemeris information for all the operational SV""s. The power-off time uncertainty, sigmaTime, is kept under one millisecond by running a real-time clock with a software-compensated crystal oscillator reference. Such information is instantly available at power-up to reduce the time necessary to produce a first fix when receiver signal levels are under xe2x88x92150 dbm even for the strongest SV.
An advantage of the present invention is that a system and method are provided for faster initialization of navigation satellite receivers.
Another advantage of the present invention is that a system and method are provided for improving the sensitivity navigation satellite receivers.